判断两直线平行

  isIntersect3(line1, line2) {
      // 转换成一般式: Ax+By = C
      let a1 = line1.endPoint.y - line1.startPoint.y;
      let b1 = line1.startPoint.x - line1.endPoint.x;
      let c1 = a1 * line1.startPoint.x + b1 * line1.startPoint.y;

      //转换成一般式: Ax+By = C
      let a2 = line2.endPoint.y - line2.startPoint.y;
      let b2 = line2.startPoint.x - line2.endPoint.x;
      let c2 = a2 * line2.startPoint.x + b2 * line2.startPoint.y;

      // 计算交点		
      let d = a1 * b2 - a2 * b1;

      // 当d==0时，两线平行
      if (d == 0) {
        return false;
      } else {
        var x = (b2 * c1 - b1 * c2) / d;
        var y = (a1 * c2 - a2 * c1) / d;
        // 检测交点是否在两条线段上
        if ((isInBetween(line1.startPoint.x, x, line1.endPoint.x) || isInBetween(line1.startPoint.y, y, line1.endPoint.y)) &&
          (isInBetween(line2.startPoint.x, x, line2.endPoint.x) || isInBetween(line2.startPoint.y, y, line2.endPoint.y))) {
          return true;
        }
      }
      return false;
    },